In co-pending application U.S. Ser. No. 031,368 and co-pending application U.S. Ser. No. 031,369 both filed on Apr. 19, 1979, both of which are hereby incorporated by reference, processes and apparatus are disclosed for drying and preheating free water-containing, glass batch pellets to an elevated temperature prior to melting. Such processes and apparatus are especially, uniquely adapted to glass manufacturing wherein the pellets are hydrologically unstable; i.e., pellets which, when attempted to be dried and preheated in a single processing operation in a bed, by passing a heating and drying medium therethrough which is wet, or humid, convert into process-disabling aggregates. Such aggregate formation results when using, for example, flue gases emanating in a fossil fuel fired melter which have been passed through a heat exchanger as, for example, a regenerator or a recuperator. Such instability also results when using combustion products having a wet bulb temperature substantially the same as the wet bulb temperature of such flue gases, for example, a wet bulb temperature of between about 130.degree. F. to about 140.degree. F. The aggregate formation problem is solved, in such applications, by forming separate beds of the free water-containing glass batch pellets in a preconditioning chamber and then preconditioning those beds to a hydrologically stabilized state by removing at least some of the water therefrom, after which the beds preferably in a cyclic and sequential manner, are discharged to a main vertical bed where they are then preheated to an elevated temperature. The preheating is accomplished by the use of flue gases from a fossil fuel fired melter or by separately provided gaseous combustion products with the gases, after passage through the vertical bed being used as the heating medium in the preconditioning beds.
It has been observed that after free water-containing glass batch pellets of such compositions have been produced on a rotary disc pelletizer, the surface of such pellets are sticky, or tacky, which complicates the handling of the pellets in conveying them into the preconditioning chamber for preconditioning. This problem is especially acute in instances where the glass batch formulation, on a theoretical dry oxide basis, contains between about 5% to about 20% or 25% by weight or Na.sub.2 O and especially when the Na.sub.2 O content is between about 10% and about 20% by weight. Such pellets usually contain about 5% to about 20% by weight of free water, with the tackiness being quite acute when the water content is between about 10% to about 20% (dry basis). The present invention solves this problem and satisfies the need in the art by enhancing the ability to confidently and reliably handle such type pellets at the high production speeds needed for a competitive glass manufacturing process.
Thus, in accordance with one feature of this invention, there is provided an improvement in processes for drying and preheating free water-containing hydrologically unstable glass batch pellets which processes comprise combining glass batch ingredients and water on a rotary disc pelletizer so as to form pellets, heating said pellets in a bed so as to form heated dry pellets, conveying such heated dry pellets to a glass melter and melting the heated pellets therein. The improvement comprises coating the surface of said pellets prior to heating by combining said pellets and dry, particulate glass batch ingredients having substantially the same glass forming composition as the batch ingredients used to form the pellets with a tumbling motion in a rotary drum so as to form a substantially uniform surface coating on said pellets. The combining of the free water-containing pellets will be done with a small, effective, tackiness-precluding amount of the dry glass batch ingredients. Outstanding results are achieved when the amount is about 1 to about 5 parts by weight of the dry batch ingredients per about 100 parts by weight of the free water-containing glass batch pellets.
In accordance with another feature of this invention, there is provided an improvement in processes of the type which comprise supplying an admixture of dry particulate glass forming batch ingredients to a rotary disc pelletizer, introducing water to said rotating disc pelletizer and combining the batch ingredients and water thereon into pellets; the improvement comprises conveying pellets after formation on said pelletizer to a rotating tubular drum, diverting a portion of the supplied admixture and conveying said diverted portion to said rotating tubular drum without discontinuing the supply to the pelletizer, maintaining tumbling contact between the pellets and diverted portion in said drum for sufficient period of time to uniformly coat the pellet surfaces with said diverted dry particulate glass batch and discharging coated pellets from the drum. In this way, the surface of the pellets are provided with a coating which effectively partially dehydrates the pellet surface such that the surface has a greatly decreased adhesive character, thereby allowing for more reliable subsequent handling and processing.